A Limbal Relaxing Incision (LRI) is a deep circumferential incision made in the peripheral margin of the cornea (the limbus) to reduce the astigmatic component of a patient's refractive error. The incisions are most often performed at the time of cataract surgery, but can also be performed as a separate procedure.
Accurately making a limbal relaxing incision can be technically challenging. One challenge is making an incision that is both smooth and at a proper angle. The difficulty is that a surgeon must incise an arc concentric about an axis of the eye over an angle of 90 degrees or more while maintaining the tangential orientation and angle of inclination of the keratome (the scalpel which performs the incision to a preset depth) as it travels around the limbus. A second challenge is ensuring the incision is at precisely the correct length, since the degree of astigmatic reduction is determined precisely by the length of the incision. The difficulty here is that due to differences in the corneal diameter from patient to patient, the same number of degrees of arc can result in significant differences in the absolute length of the incision.
Corneal markers are available to surgeons to pre-mark the cornea in order to assist in achieving an accurate incision. One type marks radial lines on the cornea at 40 degrees and 80 degrees, but the surgeon must still estimate the actual incision endpoints for an individual patient. Other corneal markers mark a circumferential line, but the surgeon must still estimate the circumferential length to accommodate for the individual patient's cornea size.
Commercially, Mastel Instruments offers a Mendez gauge-style ring that serves as a guide for making limbal relaxing incisions. However, even with the gauge the surgeon still must estimate incision length. With the gauge there is also a risk of overshooting the desired incision length since friction can develop between the guide and the keratome, causing the guide to stick. Another commercial provider, Duckworth and Kent, makes a similar ring gauge of different diameters, but this gauge also does not eliminate the risk of overshooting the desired incision length.